DSM r11.2 uses MDB version 1.0.4 and Service Desk r12.1 uses MDB version 1.5.
For DSM and Service Desk to share the MDB, the DSM MDB should be upgraded from version 1.0.4 to 1.5.
Run the Service Desk r12.1 installer on DSM r11.2 remote MDB server and click choose the "Install MDB" setup option. This would upgrade the MDB and creates the Service Desk database user. Make sure to enable the 'Compatibility Mode' option in the Database Configuration screen as shown below:
This would create MDB version 1.5 with appropriate r12.1 Service Desk tables. Once the upgrade is complete, verify the MDB version by running the following SQL query:
Select * from mdb_version
The result should show as below:
Once the installation is complete, DSM r11.2 should be able to work with MDB version 1.5. You may now proceed with the installation of Service Desk r12.1.